A conventional exhaust gas treatment system will be described with reference to FIG. 12.
Combustion exhaust gas sent from a boiler 111 is introduced into a denitrification apparatus 114. In a flow path to the denitrification apparatus 114, an ammonia injection apparatus 112 is provided to inject NH3, which is supplied from an ammonia tank 113, into the exhaust gas. For the exhaust gas into which NH3 is injected, NOx is converted into nitrogen in the denitrification apparatus 114 by selective reduction reactions expressed in Equations (1) and (2), by which denitrification is accomplished.4NO+4NH3+O2→4N2+6H2O  (1)NO+NO2+2NH3→2N2+3H2O  (2)
The desulfurized exhaust gas passes through an air heater 116 and a heat exchanger 117, and flows into an electrical dust collector 118, where soot and dust are removed. Thereafter, SO2 is removed in a wet type desulfurization apparatus 119. The coal or heavy oil burning exhaust gas contains hazardous trace substances such as mercury. Among these substances, mercury is generally difficult to remove. It is thought that mercury exists in exhaust gas mainly as metallic mercury (Hg) or mercury chloride (Hg2Cl2 or HgCl2). In the case where mercury exists in a form of Hg2Cl2 or HgCl2 in this treating process, mercury is removed by the wet type desulfurization apparatus 119 because Hg2Cl2 or HgCl2 in the exhaust gas is dissolved in water by gas-liquid contact in the desulfurization apparatus 119. However, in the case where mercury exists in a form of metallic mercury, the removal rate of mercury is low, and mercury is discharged from a stack 121 through a heat exchanger 120 without being removed because metallic mercury is difficult to dissolve in water. In the heat exchanger 120, the combustion exhaust gas whose temperature has been decreased is heated by thermal energy recovered by the heat exchanger 117 at the preceding stage of the wet type desulfurization apparatus 119.
To solve the above problem, the inventors of the present invention found that metallic mercury in exhaust gas can be converted into HgCl2 on a denitrification catalyst by a reaction expressed by Equation (3) in the presence of HCl. As the result, in Japanese Patent Provisional Publication No. 10-230137 (No. 230137/1998), the applicant has proposed a method in which on the upstream side of the denitrification apparatus 114, metallic mercury is converted into a chlorine compound having high water solubility on a denitrification catalyst by adding a mercury chlorinating agent such as HCl to exhaust gas containing mercury, by which mercury is removed with high efficiency by the wet type desulfurization apparatus 119 provided on the downstream side of the denitrification apparatus 114.Hg+HCl+½O2→HgCl2+H2O  (3)
In this method, however, since the service temperature of the denitrification apparatus 114 is limited to 300 to 450° C., for a fuel having a low Cl content, it is necessary to add excess chlorinating agent (HCl etc.) suso that mercury chloride is stabilized. However, the addition of chlorinating agent more than needed causes corrosion of gas flues and downstream-side apparatuses in the system, and finally may shorten the life of the plant facility.
Thereupon, the applicant of the present invention has proposed, in Japanese Patent Provisional Publication No. 2003-53142, an exhaust gas treatment method and an exhaust gas treatment system, in which after metallic mercury is oxidized into mercury chloride at a reaction temperature not higher than 300° C. by a solid catalyst, mercury is removed by an alkali absorbing solution, by which even for a fuel having a low Cl content, metallic mercury can be oxidized stably into mercury chloride without the addition of a chlorinating agent (HCl etc.).
FIG. 13 shows a configuration of an exhaust gas treatment system disclosed in Japanese Patent Provisional Publication No. 2003-53142.
In the exhaust gas treatment system shown in FIG. 13, on the downstream side of a boiler 231, a denitrification apparatus 232, an air heater 233, a dust collector 234, a heat exchanger 235 for recovering thermal energy, a catalytic oxidation apparatus 236, a wet type desulfurization apparatus 237, a reheater 238, and a stack 239 are arranged in succession. Also, this exhaust gas treatment system has a temperature controller 240 to which a gas temperature at an inlet a of the catalytic oxidation apparatus 236 and a Hg concentration at an outlet b thereof are sent as signals, and a heat medium flow rate regulating valve 241 to which a flow rate regulation signal is sent from the temperature controller 240.
The exhaust gas treatment system shown in FIG. 13 is configured so that the gas temperature of the catalytic oxidation apparatus 236 is controlled so as to be an optimum temperature by regulating the quantity of a heat medium supplied to the heat exchanger 235 provided on the upstream side by means of the gas temperature at the inlet a of the catalytic oxidation apparatus 236 and/or the concentration of metallic mercury or mercury chloride at the outlet b thereof. When the gas temperature is detected and controlled, the gas temperature at the inlet a of the catalytic oxidation apparatus 236 is detected, the detected temperature is compared with a reference temperature preset according to the type of fuel in the temperature controller 140, and a flow rate regulation signal is sent to the heat medium flow rate regulating valve 241 according to the change amount. By the regulation of the heat medium flow rate regulating valve 241, the quantity of heat medium supplied to the heat exchanger 235 is changed, and the gas temperature at the outlet of the heat exchanger 235 is controlled so as to be a proper temperature.
Thus, the exhaust gas treatment system shown in FIG. 13 has the catalytic oxidation apparatus 236 which oxidizes metallic mercury in combustion exhaust gas containing SOx and mercury into mercury chloride at a reaction temperature not higher than 300° C. by a solid catalyst, the wet type desulfurization apparatus 237 which removes mercury by means of an alkali absorbing solution, the temperature controller 240 which controls the reaction temperature when metallic mercury is oxidized into mercury chloride, and the like. Therefore, the gas temperature at the outlet of the heat exchanger 235 can be controlled so as to be a proper temperature, so that even for a fuel having a low Cl content, and without the addition of a chlorinating agent (HCl etc.), metallic mercury can be oxidized stably into mercury chloride, by which metallic mercury in exhaust gas can be removed effectively.